A TFT back plane is a device wherein a plurality of thin film transistors (TFT) is arrayed on a substrate. Presently, an active matrix liquid crystal display and an active matrix organic electroluminescent display (organic EL display, organic light emitting diode display) using the TFT back plane are actively studied and developed. As the TFT, a hydrogenated amorphous silicon (a-Si:H) TFT and a low-temperature polycrystalline silicon (LTPS) TFT have been used.
In such a TFT back plane, the improvement of threshold voltage stability and the improvement of in-plane uniformity are critical issues. For threshold voltage stability, the suppression of change in the threshold voltage of the TFT caused by various reasons (change with time, electrical stress, and thermal stress) is a critical issue. On the other hand, for the in-plane uniformity of the transistor array, the suppression of threshold voltage fluctuation between elements caused by the non-uniformity of fabricating processes and material structures is a critical issue.
The stability of threshold voltage will be further described exemplifying an a-Si:H TFT, which is an n-channel TFT. When a positive gate-source voltage (Vgs) is supplied to the a-Si:H TFT, electrical stress is caused, and the threshold voltage elevates. When a negative Vgs is supplied, the threshold voltage lowers. When the threshold voltage is varied by causes, such as electrical stress, and exceeds the control power voltage range of Vgs, the TFT does not operate. This time can be considered to be one definition of lifetime of the TFT.
Next, the in-plane uniformity of the transistor array will be further described exemplifying an LIPS TFT array. Since the material of the channel layer is a thin polycrystalline film, if a plurality of LIPS TFT is fabricated on the same substrate, the density of an electrically active defect in the channel layer of each TFT is different from one another. As a result, the fluctuation of threshold voltages is caused between TFTs. Also when in-plane non-uniformity is present in the crystallizing process of the channel layer, the fluctuation between elements is caused. If such fluctuation of the threshold voltage is present in the TFT back plane, the display uniformity and the gray level of the display is impaired.
As a new TFT that can solve such issues, a TFT using an oxide semiconductor in the channel layer recently attracts attentions. For example, a method for fabricating TFT using a thin film of amorphous In—Ga—Zn—O (IGZO) formed by an RF sputtering method as the channel layer is disclosed in Yabuta et al., Appl. Phys. Lett., 89, 112123 (2006).
Also in an oxide TFT, changes in the threshold voltage due to one of electrical stress and the combination of electrical stress and thermal stress has been observed. Such changes are disclosed in Riedl et al, Phys. Stat. Sol., 1, 175 (2007) and Kim et al., International Electron Device Meeting 2006 (IEDM'06), 11-13, 1 (2006). Furthermore, changes in threshold voltage in an oxide semiconductor TFT by irradiating visual light and ultraviolet light is disclosed in Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006).